The present invention relates in general to simulating a metal stamping process, and, more specifically, to providing a two-dimensional modeling of the restraining forces created by a drawbead that is accurate and computationally efficient.
The purpose of a drawbead in a typical stamping process is to provide a restraining force that helps control material flow when a metal sheet or blank is deformed into the shape of the die. There are two stages in a typical draw (i.e., stamping) process: binderset and die closure. In binderset, upper and lower blankholders close up against the blank to initiate the restraining force. In die closure, the blank is drawn or punched into the die cavity and deformed into the shape of the die. A drawbead consists of a male and a female side that is mounted separately to upper and lower holders. When the two holders move to a closed position in the binderset phase, the two sides of the drawbead engage the blank and then deform the metal sheet into the bead. The drawbead then remains fully engaged during the die closure phase. As the blank is forced into the die cavity, the metal flows through the drawbead. The sheet metal undergoes stretching and bending deformations, moving against friction to create a restraining force acting on the metal flow.
The restraining force generated by a drawbead changes throughout the entire binderset process. It reaches its maximum as the drawbead becomes fully engaged with sheet metal all around. Thus, the restraining force ramps up to a maximum value at the end of the binderset phase, and it keeps this value throughout the die closure phase until the blank edge (i.e., outline) moves into the drawbead. At that point, the force decreases according to the portion of the metal sheet still engaging the drawbead. Because of the desire to keep material utilization high and minimize scrap, stamping processes are often designed so that the blank outline partially or completely flows into and through the drawbead.
When developing a stamping process and the tooling and the metal blanks to be used, various computer aided engineering (CAE) tools are often used to analyze candidate designs and to optimize them. One particular example of a method and apparatus for analyzing a stamping process is shown in U.S. Pat. No. 5,379,227, entitled “Method for Aiding Sheet Metal Forming Tooling Design,” which is incorporated herein by reference in its entirety. It is imperative for CAE engineers to accurately simulate the forces acting during the stamping process in order to properly choose an initial blank design that results in a desired final stamped shape while minimizing the outline of the blank. In conventional models, a line bead has been used to simulate a real drawbead due to its computational efficiency over a full three-dimensional model. In the line bead model, a drawbead centerline and its strength have been used to define a real drawbead's location and its maximum restraining force. This model remained fixed during a complete simulation. The prior models fail to simulate the force changes either during initial drawbead engagement or during movement of a blank edge into the drawbead. It would be desirable to simulate these force changes while remaining computationally efficient.